The present invention relates to a pulverized magnetic material capable of being incorporated into a non-magnetic matrix such as rubber or plastic material at a high concentration and being oriented to a high degree to form a rubber or plastic magnet of high energy product. The present invention is also directed to a rubber or plastic magnet produced from the pulverized magnetic material.
Heretofore, a rubber magnet or a plastic magnet has been widely used in many applications because of its desirable properties, especially good plasticity or resiliency, superior workability, etc. which are not the case in hard magnets such as sintered ferrite magnets, alloy magnets, etc. However, due to the fact that the rubber or plastic magnet was produced by blending a pulverized magnetic material with a rubber or plastic matrix, the magnetic properties of thusly produced rubber or plastic magnet were not necessarily satisfactory and accordingly its applications have been restricted. For example, it is necessary to employ a magnet of a much larger size than that of the conventional sintered magnet for the same application and thus the development of the rubber or plastic materials has been hindered.
The critical factors for improving magnetic properties of the rubber or plastic magnet are some physical properties of the magnetic powder to be incorporated in the rubber or plastic magnet in a quantity exceeding about 90% by weight. The properties of the magnetic powder must primarily meet the following two requirements with respect to the matrix.
(1) The magnetic powder can be filled in the rubber or plastic matrix as much as possible.
(2) The particles of the magnet powder can be easily oriented in the rubber or plastic matrix in one desired direction.
A typical example of the conventional magnetic powders which have been successfully utilized for producing a rubber or plastic magnet is of the magnetoplumbite type. However, the magnetic powder of this type has not fully satisfied these two requirements. Rather, these two requirements are not compatible with each other for the conventional magneto-plumbite powder. More specifically, those powders which are capable of being filled at a high concentration are not easily oriented, while those powders which are capable of being easily oriented in one direction are not easily filled.
It is known that as particle size decreases, the coercive force of a ferromagnetic powder increases and reaches a high value when the particle size becomes single-domain size (1.mu. or less). It is also known that single-domain size particles of about 1.mu. are not very well oriented in a strong magnetic field because the orientation effect decreases with the decrease of the particle size due to the decreased torque applied to such small particles. Magnetic properties of a ferromagnetic powder depend also on the crystalline structure of magnetic particles. Single-domain particles have a high coercive force as mentioned above but the magnetic orientation effect in a plastic or rubber matrix is lower for polycrystal particles than for single crystal particles. Multi-domain particles (larger than 1.mu.) exhibit various magnetic proparties depending on the crystalline structure. If such particles consist of single crystals, the coercive force is very low due to the free movements of the Block walls upon application of a magnetic field but the magnetic orientation effect is the greatest because of the fact that the magnetic domains in each particle develop in one direction so that the torque exerted on the magnetic particles by the magnetic orientation field becomes large. On the other hand, polycrystal particles having single-domain size crystallites have a high coercivity as the movement of the Block walls is blocked by the interfaces between the crystallites. However, such particles are not easily oriented under a strong magnetic field due to the random orientation of the crystallites. The only exception is the case where each polycrystal particle consists of single-domain crystallites which have been unidirectionally aligned (see U.S. Pat. No. 3,764,539).
As for the mechanical properties, particularly the filling ability of a magnetic powder and the workability of a plastic or rubber magnet, it is known that the larger the particle size, the higher are the mechanical strength and workability of a plastic or rubber magnet because of the better filling ability of the particles in the rubber or plastic matrix. Although single-domain particles have a high coercive force, they are not desirable from the mechanical criteria. The magnetic properties depend also on the filling ability which determines the remnance.
Roughly, a rubber or plastic magnet is conventionally produced according to two methods. One method is that a ferromagnetic powder, particularly of magnetoplumbite type such as barium ferrite, strontium ferrite or lead plumbite ferrite, having an average particle size over 1.5.mu. (multimagnetic domain size) and polycrystal structure is incorporated into a rubber or plastic matrix. The advantage is that the magnetic powder is easily filled in the matrix whereby a rubber or plastic magnet having a good mechanical strength and a good workability is easily obtained. However, such magnet has a drawback by the fact that the magnetic powder dispersed in the matrix cannot be oriented by magnetic orientation procedure, resulting in a low energy product (BHmax). Accordingly, it has been believed in the art that a superior flexible rubber or plastic magnet can only be obtained by blending a magnetic powder having single domain size, i.e. less than 1.mu., usually 0.1-1.0.mu. with a rubber or plastic matrix and then subjecting the mixture to magnetic orientation procedure. The advantage of this second method is of course that the magnetic powder has a high coercive force and a relatively good (but not enough) orientation effect is obtained thereby to improve the remnant magnetic flux, but the drawback is that it is difficult to fill the magnetic powder in the matrix at a high concentration, resulting in a low remnant magnetic flux and the improvement in the energy product is not satisfactory.
One approach to overcome these disadvantages was proposed by U.S. Pat. No. 3,764,539 in which polycrystal magnetic particles, such as barium ferrite having uniform particle sizes of approximately 5.mu. are incorporated into an elastomeric binder without use of magnetic orientation. The magnetic particles in this case are those which have been so treated with certain additives that single-domain particles are first magnetically oriented in one direction and then bonded together by the additives to form a larger particle of about 5.mu. or more. Each polycrystal particle is characterized in that the anisotropic axis of the single-domain particles (crystallites) are fixed in one direction in each polycrystal particle and the crystallites are separated by the additive layers. As described hereinbefore, it is easy for such large magnetic particles to attain a high concentration in the elastomeric binder. The magnetic properties are satisfactory but it is necesssary to prepare the anisotropic polycrystal particles by a very complicated procedure. U.S. Pat. No. 4,022,701 proposed to overcome the difficulties in the prior art by adopting a special plastic binder system. More specifically, magnetic particles such as barium ferrite having a particle size of 1.mu. or less (single domain size) are blended with metal-crosslinked copolymers of .alpha.-olefin and .alpha.,.beta.-unsaturated carboxylic acid, and then subjected to magnetic orientation. Thus, the approach of this patent relies on the properties of the copolymers. There is no disclosure of parameters which are to be satisfied by the particle sizes of the magnetic powder.
French Pat. No. 1,323,095 proposed to produce a flexible magnet wherein a magnetic powder such as barium ferrite having particle sizes of 0.5-10.mu. whose average particle size is between 1.mu. and 1.5.mu. is mixed with a plastic binder selected from special plastic materials which maintain fluidity even at a very high concentration of the magnetic powder. Accordingly, this patent again relies on the improved binder material. Magnetic or mechanical orientation is not used in the process of this patent. Also, there is no teaching on how the magnetic powder is produced.
As to the conventional methods of preparation of magnetic powders, sintered magnetoplumbite type magnetic material is ground or crushed with use of a ball mill or a vibration mill. Two procedures are presently employed, the dry method and the wet method. In order to obtain a magnetic powder having a particle size of less than 1.mu., the wet method using water or other liquid medium must be used because the dry method cannot attain particle sizes of less than 2.about.3.mu. after a long period of time. However, the particles thusly obtained by the wet method are very uniform and it is difficult to fill them into a rubber or plastic matrix.
A magnetoplumbite type magnet of high quality can be prepared by sintering a starting mixture composition of oxides of high purity at a high temperature above, for example, 1200.degree. C. However, the sintered magnet is very hard and is difficult to pulverize into single-domain sizes. The dry method can only produce 2.about.3.mu. particles as just mentioned due to the low crushing ability of the mills and the wet method must be relied on at least in the final pulverizing step. However, the wet method produces a magnetic powder having a very uniform particle size distribution which cannot meet the requirement (1) above. If a magnetic powder is produced by the dry method, the magnetic properties in a rubber or plastic magnet are poor due to the large particle sizes and the multi-domain structure. As a compromise, a rubber or plastic magnet can be prepared by sintering a starting mixture material at a lower temperature below, for example, 1200.degree. C. and sometimes using additives for lowering the sintering temperature. With the method a magnetic powder of any particle size can be easily obtained since the sintered body is easily pulverized. However, the magnetic properties are poor owing to the unreacted portion and/or the impurities.
Accordingly, a primay object of the present invention is to provide a rubber or plastic magnet whose magnetic properties are substantially improved.
Another object of the present invention is to provide a rubber or plastic magnet which comprises a rubber or plastic matrix and a magnetoplumbite type magnet powder dispersed therein at a high concentration and with a high degree of orientation.
A further object of the present invention is to provide a magnetic powder which is adapted to produce a rubber or plastic magnet of high magnetic properties.
A still further object of the present invention is to provide a magnetic powder of magnetoplumbite type which is capable of being incorporated in a non-magnetic matrix such as rubber or plastic material not only at a high concentration but also with a high degree of orientation.